End mills designed for shouldering applications, i.e. milling around an external periphery of a workpiece, typically have a maximum effective cutting length of twice the diameter of the end mill (hereinafter “2D”; with similar length dimensions being represented similarly, e.g. two and a half times the diameter will be written as “2.5D” etc.). Unless stated to the contrary, references to end mill diameter in the specification and claims refer to a diameter of the cutting portion at the cutting end face.
While in theory end mills can have any effective cutting length, in practice it is exceedingly rare to find end mills that can mill effectively at a depth greater than 2D. This is because increasing depth exacerbates vibration of the end mill reducing both work piece surface finish and end mill tool life to standards lower than those accepted by industry. To elaborate, end mills bend during shouldering, since the end mill is only held at one end thereof and the other end thereof is forced against and impacts a rigidly held workpiece. Such impacts also cause a rebound type effect, with this effect being comparatively greater with increased end mill length.
Similarly this effect is also more severe when milling comparatively harder workpiece materials since the milling forces exerted on the end mill are greater with each impact on the hard workpiece.
Another problem which is exacerbated with increased depth of milling is chip evacuation. To elaborate, large flute depth is most critical near the cutting end face since each chip is first contacted by the tooth starting near the cutting end face as it first enters the material. The chip remains within the flute near the cutting end face for comparatively more time than the remainder of the flute, since it only exits the flute when the end mill has rotated fully in the work piece and the flute exits the workpiece allowing the chip to be ejected. While not being bound by theory, if the flute is insufficiently sized to contain the chip, abutment of the chip (which protrudes from the insufficiently sized flute) against the end mill and workpiece can increase vibration and even cause end mill breakage. It will be understood that with increased distance from the secured shank portion of the end mill this effect will be more significant.
Due to the high performance requirements in today's industry, what was once considered insignificant changes to end mill design are now able to define whether an end mill is acceptable to industry standards of surface finish and chip removal or not. While any end mill can theoretically machine any material, only by providing a competitive tool life for a given material removal rate and a desired level of surface finish can an end mill actually be considered relevant for a particular application.